This invention generally relates to self-oscillating circuits and more specifically is directed to a free-running circuit particularly adapted for providing a periodic, pulsed output voltage.
In general, video information is displayed by a television receiver or a raster which is scanned horizontally at a first rate and scanned vertically at a second, generally slower rate. The received video information is presented as amplitude-modulated synchronizing pulses by which the raster scanning of the television receiver is synchronized with the information to be viewed. For proper picture framing, it is required that the horizontal sweep system be synchronized in frequency and phase of oscillation with the horizontal synchronizing signal transmitted from the broadcast station. This synchronization requirement is applicable not only in television receivers where the standardization of television waves establishes a predetermined relationship between horizontal and vertical synchronizing signals, but also in a video display as used in a computer terminal or in a data display presentation system which may be required to interface with a great variety of input synchronization signals.
Deflection circuits utilized in television receivers, and in CRT video displays in general, synchronize the deflection signals used to control the sweep of the electron beam therein with synchronizing pulses recovered from the composite video signal received by the television receiver or generated in the video display. The synchronized signals are typically generated by the charge-discharge cycle of a capacitor in generating a sawtooth current waveform having a predetermined period and magnitude. The ramp of the sawtooth current waveform is generally developed from the discharge of a capacitor while the capacitor is recharged during the retrace period. This sawtooth current waveform is applied to the CRT's deflection coils in causing the electron beam to sequentially and repetitively scan and retrace over the face plate of the CRT at the appropriate times.
The prior art discloses various approaches to deflection circuit design and, in particular, synchronization oscillator design to achieve synchronization of electron beam sweep with input synchronization pulses. Early attempts in this area utilized switching diodes in combination with a voltage source to alternately charge and discharge a capacitor. Later efforts employed switching transistors in CRT sawtooth current waveform generation circuits which resulted in improvements in switching speeds and power consumption. Still later work in this area gave rise to the development of silicon controlled rectifier (SCR) circuits formed of a semi-conductor assembly controlled by signals of small magnitude applied to a control electrode, or gate, and capable of operating at higher currents than that of normal rectifiers. The transistor and SCR CRT drive circuits, which generally took the form of multivibrator circuit combinations, were not without limitations. Transistorized multivibrators tended to be overly complicated while SCR oscillators suffered from instabilities, or drift, in the signal voltage levels required to initiate the transition to a stable oscillating state as well as requiring an outside source of high power signals to terminate the SCR's oscillatory state.
One example of an oscillator employed in the horizontal drive circuit of a video display is disclosed in U.S. Pat. No. 4,263,615 to Beaumont and Steinmetz. In this approach a variable time delay monostable multivibrator is triggered by the leading edge of the horizontal drive pulse, the clocked output signal of which is coupled to a precision astable multivibrator. Potentiometer adjustment of the monostable multivibrator provides for adjusting video information position with respect to raster scan while the astable multivibrator acts as the oscillator in synchronizing horizontal sweep circuitry to the horizontal input drive signal. The astable multivibrator is a free-running oscillator which oscillates at whatever frequency it is designed for until it receives an input synchronization signal, at which time it locks onto the frequency of the input synchronization signal which may be different that its original frequency. U.S. Pat. No. 4,253,117 to Kadlec discloses a system for increasing synchronization signal injection to a free-running multivibrator in the horizontal drive circuit of a video display for enhancing synchronization signal frequency capture range. By increasing sync signal frequency capture range, this system permits a video display such as used in a computer terminal or a data display presentation system to interface with a great variety of input sync signals. Another example of an oscillating circuit utilized in a video display is provided in U.S. Pat. No. 4,234,828 to Matthews wherein is disclosed an SCR-analogue dual coupled transistor vertical oscillator for synching the vertical sweep in a video display with a vertical synchronization input signal. This approach makes use of a coupled transistor configuration in combination with a capacitor for generating a precisely defined sawtooth voltage waveform for controlling vertical sweep and flyback with stable, free-running oscillation availabe at two, variable DC levels. The aforementioned systems involve the use of a multi-transistor multivibrator arrangement or a multi-transistor SCR analogue circuit arrangement for providing an oscillating output in response to a synchronization signal input.
The aforementioned self-oscillating circuits are responsive to sync signal inputs for driving a high voltage supply in the video display for controlling electron beam intensity and position therein. The high voltage power supply typically includes an isolation transformer. An example of a power supply designed for use in a television receiver is disclosed in U.S. Pat. No. 3,845,352 to Newman et al wherein the vertical deflection windings of the television receiver are coupled directly to the output of a push-pull amplifier comprising a complementary pair of electronic devices. Bipolar voltages for driving the complementary pair are derived from horizontal scanning signals by a pair of oppositely-poled secondary windings on the horizontal output transformer, or high voltage power supply. The unregulated high voltage input is thus controlled by the horizontal drive system for providing appropriate timing signals to horizontal deflection circuitry for controlling electron beam position on the face plate of the CRT. U.S. Pat. No. 4,261,032 to Cavigelli discloses a self-oscillating, high voltage DC power supply for a CRT. A charging circuit for an oscillator coil within the high voltage power supply is provided by means of a DC supply and a switching transistor connected between the coil and ground. A feedback coil inductively coupled to the oscillator coil and wound in the opposite direction is incorporated in the base drive circuit of the transistor switch. The feedback coil operates to open the switch by means of a current induced in the base drive circuit when the current in the feedback circuit reaches a predetermined level related to the current in the oscillator coil primary in regulating transistor operation.
The self-oscillating circuits and sawtooth generating high voltage supplies described above all make use of a plurality of inductively coupled transformer coils and/or multi-transistor multivibrating circuits. The present invention is intended to eliminate the complexity and expense of these approaches by providing a self-oscillating circuit comprised of a single transistor and a pair of isolated coils, one of which may be utilized as the primary of a high voltage sweep transformer to drive the CRT of a video display.